Caustic-free, thermosetting phenol-formaldehyde resins have been prepared by combining one mole of phenol with more than one mole of formaldehyde in the presence of a suitable catalyst such as CaO or MgO. These resins are water-miscible in the initial stages of the reaction, but become progressively less miscible with water as the reaction proceeds. On still further polymerization, the resins become water-insoluble. Whether these resins are made with small or large average size molecules, they suffer from the extreme disadvantage of very limited storage lives at usual ambient temperatures so much so, that they have to be often stored and shipped under refrigeration.
Chemically, these caustic-free phenol-formaldehyde resins may be described as a series of phenolic rings joined by methylene linkages, randomly at ortho or para positions, relative to the phenolic hydroxyl group, and having reactive terminal methylol groups, also randomly ortho or para to the phenolic hydroxyl.
The structure of these resins may be represented by the following formula I: ##STR1##
It is also known to form a thermosetting phenol-formaldehyde resin of a different type by reaction of one mole of phenol with one or more moles of formaldehyde in the presence of a catalyst in an aqueous reaction medium, the catalyst being a salt of a monocarboxylic acid with a metal of the group of Transition Elements of the Periodic Chart of Elements, hereinafter referred to as a "metal carboxylate catalyst".
The catalysts employed may be salts of monocarboxylic acids with metals selected from the group of Transition Elements of the Periodic Chart of Elements, such as zinc, manganese, cobalt, nickel, iron, chromium and the like. Suitable acid moieties of this catalyst may be selected from the group of formic acid, acetic acid, propionic acid, caproic acid, caprylic acid and capric acid and others. The preferred salts are zinc acetate and manganese acetate.
The amount of formaldehyde used may be in the range from 1 mole to 3 moles per mole of phenol. A preferred mole ratio is in the range 1.5 moles to 2.2 moles of formaldehyde per mole of phenol. The amount of catalyst used may be varied over a large range but quantities as low as 0.02 to 0.2 mole ratios are sufficient.
These prior art resins are produced by a two-stage reaction, in which an initial exothermic stage is carried out at a temperature of from about 60.degree. to about 80.degree. C. until heat output ceases, and then the temperature is raised to about 90.degree. to about 100.degree. C. and maintained thereat until the desired degree of polymerization is achieved.
These latter prior art resins are characterized by the presence of dibenzyl ether linkages of the type: ##STR2## and the resin molecules consist primarily of ortho-ortho substitution.
These prior art resins are thermosetting and form infusable insoluble brittle masses on the application of heat, accelerated, if desired, by the addition of small quantities of organic or inorganic acids, for example, benzene sulfonic acid, toluene sulphonic acid or sulfuric acid. The resins are characterized by an infra-red spectrum which displays large absorption peaks at wave members of 760 cm.sup.-.sup.1 (ascribed to ortho-ortho substitution), 1010 cm.sup.-.sup.1 (ascribed to methylol groups), 1050 cm.sup.-.sup.1 (ascribed to the ether groups) and 1230 cm.sup.-.sup.1.
While the above formula (II) is ascribed to these prior art resins, as will become more apparent below, the end groups do not appear to be wholly methylol groups but contain hemiformal groups.